Field of the Invention
The invention concerns the field of the manufacture of semifinished products such as rolling ingots and extrusion billets made from aluminium alloys by vertical semicontinuous casting.
More precisely, the invention concerns a direct-cooling device and method, with a double row of jets, providing gradual and continuous quenching of the product during solidification, and in particular during the phase where the casting starts, so as to control and minimise the phenomenon of butt curling, and allowing subsequent hot rolling, or extrusion, without prior sawing of the ingot foot, and this without tearing or cracking.
The ingot mould may or may not comprise, on the working surface thereof, a graphite insert in order to improve the surface finish in permanent regime.
The products may be intended for the manufacture of any application in the form of rolled sheets, strips, profiles or forged parts obtained by extrusion.
Description of Related Art
Rolling ingots and extrusion billets are typically manufactured by casting in a vertical mould, or ingot mould, positioned on a casting table above a casting pit.
The mould has a rectangular cross section in the case of ingots or circular in the case of billets, with open ends, with the exception of the bottom block at the start of casting positioned so that it allows initial filling of the mould before being moved down by an elevator system, while the mould continues to be fed by the top end. The mould and false bottom define the cavity in which the metal is cast.
At the start of the casting process, the bottom block is situated in its highest position in the mould. As soon as the metal is poured and cooled, typically by means of water, the bottom block is lowered down at a predetermined speed. The solidified metal is then extracted through the bottom part of the mould and the ingot or billet is thus formed.
This type of casting, in which the metal extracted from the mould is cooled directly by the impact of a cooling liquid, is known by the term semicontinuous casting, typically vertical, with direct cooling, or VDCC for vertical direct chill casting.
In semicontinuous casting, the difficulty lies in the success of the change from zero speed at the start of formation of the product to the permanent-regime or steady state speed. This change results in a deformation of the ingot base, known to persons skilled in the art by the term butt curling. If it is too pronounced, which occurs when the base is cooled too violently, the butt curl may cause what persons skilled in the art call “bleed-out”, which may sometimes degenerate into “hang-up”, that is to say a jamming of the ingot in its mould. The butt curl associated with an unsuitable cooling regime may, less catastrophically, result in the breaking of the base or to cracks in the base. These breakages or cracks are entirely detrimental since they may propagate in permanent regime leading thereby to the scrapping of the product, or otherwise, and at the very least, they prevent the hot rolling of the ingot without sawing of the base in order to restore the integrity of the product. Finally, a butt curl that does not cause any scrapping of the casting does however result in variations in cross section of the product, which may prevent the rolling of the products without sawing of the ingot foot.
In order to limit butt curl, it is known to persons skilled in the art that it is necessary to extract less heat from the product during the start-up phase of the casting than in steady state. For this various technologies have been developed (pulsation, injection of CO2 into the start-up water, use of V-shaped ingot moulds and curved bottom blocks). The most efficient techniques consist of sufficiently reducing the cooling flow rate during start-up in order to obtain a stable film boiling regime, which extracts much less heat than the nucleated boiling regime or the streaming regime. Moreover, it is known that the rate of butt curling is an increasing function of the start-up speed, which leads to starting the casting at a speed that is generally lower than the steady state casting speed. It is therefore known to persons skilled in the art that the most important parameters are the filling speed and the casting temperature, a small extraction of heat at the beginning of the start-up phase using a sufficiently small quantity of water of suitable thermal efficiency in relation to its quality, the appropriate choice of the start-up speed with regard to the initial flow rate of water, and finally the choice, at the end of the start-up phase, of the ramping up of the casting speed and of the cooling water flow rate, which make it possible to achieve speed and cooling parameters suited to the steady state regime while guaranteeing a good soundness of the ingot foot and minimisation of curling thereof.
This can be obtained with ingot moulds known by the term “waterholes”, the interior architecture and hole diameters of which make it possible to achieve very low flow rates while guaranteeing very good uniformity of flow along the mould.
These moulds comprise either a horizontal row of holes, or two rows placed one above the other.
The application WO 2005/092540 A1 and the patents U.S. Pat. Nos. 7,007,739 B2, 5,518,063; 5,582,230 and 5,685,359 of Wagstaff Inc. disclose a sequential spray system, first of all with a first row of holes at a 22° angle of incidence, which provides the film boiling regime during start-up, and then adding above it a second row of jets issuing from holes at 45°, which end film boiling and provide sufficient cooling in permanent regime. The high differentiation between the regime with a row of jets at a low angle of incidence and the regime with spraying by the two jets, one of which has a high angle of incidence, is explicitly claimed by Wagstaff Inc.
Each of these two systems (with one or two rows as above) has drawbacks:                Moulds of the “waterhole” type with one row of holes effectively make it possible to obtain a film boiling regime with a low flow rate per unit length of mould, but they are very sensitive to the quality of the water. This is because firstly the minimum flow rate per unit length accessible with a single row of holes is not as low as when only half of the holes spray the product, as in the Wagstaff Inc. moulds sold under the names “Epsilon™” OR “LHC™” (the latter with graphite insert on the working faces). Consequently the operating point of these moulds with one row of holes is, by construction, closer to the transition to nucleate boiling, i.e. the so-called Leidenfrost point on the Nukiyama curve known to persons skilled in the art, that is to say a small variation in flow rate along the mould, or in water temperature or in water quality may easily tip the film boiling operating point towards nucleate boiling. This is why these moulds cannot be used correctly when the water is too cold, or when it is subject to seasonal variations in quality.        Sequential-cooling moulds (“Epsilon™” and “LHC™” from Wagstaff Inc.) are for their part much less sensitive to water quality since their operating point is much further from the Leidenfrost point because of the very low start-up water flow rate when only half the holes spray the product, moreover at a low angle of incidence. However, this technology has several drawbacks:        The first drawback of this technology, which explicitly claims the differentiation between the first and second spraying regimes, is the double curling phenomenon. This is because a first curling occurs at start-up with a first row of jets at an angle of incidence of 22°. Then, a second curling occurs when the jets are activated at 45°. It is necessary to know that the mechanical phenomenon of butt curling does not stop abruptly but continues to have its effects felt until late during the casting, that is to say at 1 m of cast length and more. This sequential-spraying system helps to significantly extend this transient mechanical curling regime. During the subsequent hot rolling of the ingot, this results in a risk of cracking between the first and second bowing and to the rolling rejects that result therefrom. Thus the moulds of the prior art have been optimised on the sole criterion of casting recovery and not on the behaviour under rolling of the ingot bases thus formed.        The second drawback relates to so-called but swell, prolonged because of the very low spraying rate during the first casting start-up phase.        The third drawback is the incompatibility of this technology with the casting of so-called hard alloys. This is because these are often characterised by high sensitivity to hot cracking on the one hand and by the fact that very high stresses appear therein quickly during cooling. It is essential to limit all local temperature gradients that may result in locally very high internal stresses. However, firstly the spraying phase at very low rate is propitious to hot cracking, for two reasons: the excessive time spent by the surface metal in the dangerous solidified fraction zone (the presence of a weakening residual liquid fraction) before the impact, situated very low, of the 22° jets, and the excessive spacing between the 22° jets that creates local thermal gradients propitious to the initiation of cracks, and secondly by the abrupt application of a second spraying at a high angle of incidence after the low angle of incidence regime creates precisely the conditions for appearance of a very high local thermal gradient and stresses that accompany it.The Stated Problem        
The present invention proposes to afford a solution to the problem of double curling of the ingot foot and ingot base quality, without the drawbacks that have been noted for the existing solutions, among other things and in particular for hard alloys.
It aims to optimise the start-up of the casting not only on a criterion of recovery during start-up but also on a criterion of suitability for subsequent conversion by hot rolling.
It also aims to broaden the field of applicability of all types of aluminium alloys.
It should be noted in this regard that all the aluminium alloys dealt with hereinafter are designated, unless mentioned to the contrary, in accordance with the designations defined by the Aluminum Association in the Registration Record Series that it publishes regularly.